This invention relates to a surface profile determining system and method. More particularly, this invention relates to surface profile determination relying on the reflection of optical energy applied to the surface.
The use of various kinds of industrial vision sensors has become quite widespread. Such vision sensors commonly find application in robotics systems, particularly robotics welding systems with the sensors being used to control a welding head such that it follows a weld seam. Industrial vision sensors are used for numerous other applications such as generating data describing various dimensions of a surface, which data can be conveniently sorted electronically.
An object of the present invention is to provide a vision system which determines the vertical height of a surface point by point along a line on the surface. Thus, if the surface height is measured along a coordinate Z, in a direction parallel to the direction from which the surface is observed, and a coordinate X is measured along a straight line perpendicular to the direction of Z, then the vision system provides a value of Z for each value of X along the line. This information will be described here as a surface profile, and the system will be called a profiler or profiler system.
Profilers have been proposed or built using TV cameras, linear detector arrays and flying spot camera systems. Various optical systems can be used to generate a pattern consisting, for example, of a single or multiple lines of light to be applied to a surface in order to determine a surface profile. The light pattern is projected and observed from two moderately different directions, such that changes in a dimension of the surface in the general direction of observation and projection (the Z-direction) cause an apparent shift in the pattern. The principles of triangulation can be used to determine the exact Z-dimension of each point in the pattern, thus providing one or more surface profiles.
U.S. Pat. No. 4,349,271 issued on Sept. 14, 1982, to Joseph L. Mundy, Gilbert B. Porter, III, and Thomas M. Cipolla, entitled "NON-CONTACT MEASUREMENT OF SURFACE PROFILE", discloses such a structured light profiler. This patent teaches that, by sensing the reflected light from the pattern by use of two detector arrays (one for each of two light wavelengths) one can develop a profile mapping of a complex surface.
Although such structured light profilers using TV cameras or solid state detector arrays have been generally useful, they have proven subject to several disadvantages. In particular, the sensitivity, dynamic range and background light rejection capabilities of TV and detector array systems have posed significant problems in their use in profilers. Since the directional reflectivity of angle metal surfaces with typical industrial finishes can easily vary over factors of several thousand, the dynamic range, on the order of 100 to 200:1 for standard TV camera systems, can be greatly exceeded. Additionally, the sensitivity of such detectors often requires illumination of uncooperative surfaces by a laser with sufficient power to pose laser safety hazards. A further disadvantage of many TV and array systems is that the source light level can not be modulated at high frequency, and synchronously detected to reduce sensitivity to background radiation, because TV systems employ detector elements which integrate between scans that are repeated at a relatively low frequency, such as 30 repetitions per second.
Unfortunately, a photomultiplier tube, which is a highly sensitive light detector and has relatively good dynamic range, is too expensive and cumbersome to combine into the dense array needed by most profiler configurations. On the other hand, a detector element such as a photodiode, which is less expensive and easily made small, can be used to realize a relatively low priced and convenient array, but lacks the sensitivity and dynamic range of a photomultiplier.
In the present invention the need for a detector array is avoided by using a particular type of flying spot camera. The array is replaced by two photomultipliers, or a dual photomultiplier in a single shell, thus gaining the sensitivity, dynamic range and background rejection advantages of this type of detector.
Prior art flying spot profilers typically have used a laser beam which is scanned onto a surface. The image of the beam hitting the surface is tracked by a detector array which is offset from the angle at which the beam is directed to the surface. Using optical triangulation techniques, knowledge of the angle at which the beam strikes the surface and of the position of the beam's image on the surface allows determination of the surface profile. Because such systems use solid state detector arrays, such systems are also subject to sensitivity, dynamic range, and resolution limitations found in the detector arrays used in structured light systems. In particular, the variations in surface directional reflectivity may require the use of a relatively powerful laser necessitating various special safety precautions which are economically disadvantageous and operationally cumbersome.
As a modification of the basic flying spot camera and detector array system, a recent profile measuring system, U.S. Pat. No. 4,158,507 teaches the use of a laser beam which is scanned across a surface and has its image detected by a single photomultiplier tube. In order to trace the path of the scanned beam across the surface, an optical grating is disposed between the image of the beam and the photomultiplier. As the beam sweeps across the surface, the sweep time between various opaque strips on the grating is indicative of the slope of the surface.
Although this flying spot system using an optical grating has overcome several of the disadvantages of the detector array arrangements, its measurement capability is still undesirably limited. In particular, if the grating is chosen to be quite fine (very narrow alternating transparent and opaque strips), such that high spatial resolution is obtained, the system will be able to detect small variations in surface slope. However such very narrow grating strips can produce erroneous or misleading measurements. Specifically, if there is a sharp surface change or major vertical step in the surface, a beam's image may jump over more than one stripe of the grating such that the detection system will not indicate properly the height of that step.